docs: def unique_files(event_list):
      """Find the unique files
  
      Parameters
      ----------
      event_list : list or dcase_util.containers.MetaDataContainer
          A list containing event dicts
  
      Returns
      -------
      list
          Unique filenames in alphabetical order
  
      """
  
      if isinstance(event_list, dcase_util.containers.MetaDataContainer):
          return event_list.unique_files
  
      else:
          files = {}
          for event in event_list:
              if 'file' in event:
                  files[event['file']] = event['file']
  
              elif 'filename' in event:
                  files[event['filename']] = event['filename']
  
          files = list(files.keys())
          files.sort()
          return files